Liquid dispensing systems have become an integral part of the electronics manufacturing process for depositing underfill, encapsulants, solder fluxes, surface mount adhesives, conformal coatings and other materials onto a substrate, such as a printed circuit board. Each liquid dispensing system used in the electronics manufacturing process has a particular dispensing characteristic that is determined in large measure by the desired liquid dispense pattern on the substrate, the liquid flow rate and/or liquid viscosity of the dispensed material, and the desired electronic component assembly throughput through the dispensing system.
For example, in the assembly of ball gate arrays (BGA's) and other electronic components onto a ceramic or FR-4 substrate, the component must be soldered onto the substrate to form the necessary electrical interconnections. As each component occupies a predetermined area on the substrate, the liquid dispensing system must have the capability to dispense liquid or viscous material in a controlled manner within the selected component areas. Typically, the liquid dispenser is mounted on a movable platform to provide automated and accurate movement of the liquid dispenser in three dimensions relative to the substrate with the aid of a machine vision system.
Prior to the component soldering process for establishing the electrical interconnections, it is often necessary or at least desirable to dispense a layer of solder flux onto a substrate within rectangular areas associated with each component. To provide this capability, liquid material dispensers have been developed in the past that use filled syringes or reservoirs of solder flux, and dispensing valves to dispense droplets of flux material onto the substrate in a controlled manner with up to 25,000 to 40,000 dots of fluid per hour for a typical dispenser platform. These liquid dispensers, known as "dot jetting" dispensers, are programmed to dispense an array of liquid or viscous material droplets within each selected rectangular area which are then allowed to flow into contact with each other to form a generally rectangular thin layer of flux within the component area. "Dot jetting" dispensers are also used to dispense multiple droplets of adhesive onto a substrate to adhesively bond an electronic component to the substrate at a desired location.
In the "dot jet" process, each droplet of material is formed by rapidly closing a valve against a valve seat to impart kinetic energy to the fluid material within the dispenser. Closing of the valve causes the material to break off from the nozzle of the dispenser from which it is being dispensed and propel toward the substrate and onto the substrate as a droplet. Due to the high velocity imparted to form the jet or droplet of viscous material, the jet's tail may break into smaller droplets forming satellite portions associated with the main droplet. These satellite portions have a tendency to stray or deviate from the dispensing axis and cause problems when they hit the substrate outside of the area of the main droplet. The satellite portions may fall within areas of the substrate where it is undesirable to have a coating of viscous or liquid material, and may adversely affect the uniformity of the material coating applied to the substrate. As a result, the outlet of the "dot jetting" dispenser must typically be positioned relatively close to the substrate, such as in a range of about 1.5 mm to about 2.0 mm, to limit how far the satellite portions may deviate from the dispensing axis.
However, it will be appreciated that any reduction in the height of the nozzle tip relative to the substrate has a tendency to also reduce the size of droplet that may be dispensed onto the substrate since the droplet's ability to expand in flight toward the substrate is affected. This results in the need for more droplets of material to be dispensed to cover a given area of the substrate, and also results in increased dispense cycle times and reduced throughput of components through the dispenser.
Thus, there is a need for a liquid dispensing system that dispenses droplets of liquid or viscous material in a more accurate and repeatable manner. There is also a need for a liquid dispensing system that improves control of the dot shape and application of the dots toward a substrate. There is also a need for a liquid dispensing system that reduces dispense cycle times and improves throughput of components through the dispenser over known dispensing systems.